The Collaborative International Dictionary of English v.0.48:

Observant \Ob*serv"ant\, a. [L. observans, -anits, p. pr. of
   observare: cf. F. observant. See Observe.]
   1. Taking notice; viewing or noticing attentively; watchful;
      carefully attentive; as, an observant spectator; observant
      habits.
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            Wandering from clime to clime observant stray'd.
                                                  --Pope.
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   2. Submissively attentive; obediently watchful; regardful;
      mindful; obedient (to); -- with of, as, to be observant of
      rules.
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            We are told how observant Alexander was of his
            master Aristotle.                     --Sir K.
                                                  Digby.
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The Collaborative International Dictionary of English v.0.48:

Observant \Ob*serv"ant\, n.
   1. One who observes forms and rules. [Obs.] --Hooker.
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   2. A sycophantic servant. [Obs.]
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            Silly ducking observants,
            That stretch their duties nicely.     --Shak.
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   3. (R. C. Ch.) An Observantine.
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WordNet (r) 3.0 (2006):

observant
    adj 1: paying close attention especially to details
    2: quick to notice; showing quick and keen perception [syn:
       observant, observing]
    3: (of individuals) adhering strictly to laws and rules and
       customs; "law-abiding citizens"; "observant of the speed
       limit" [syn: law-abiding, observant]